Reflection of a signal from a metallic surface which is in linear motion will produce a constant Doppler shift of the reflected signal. As a practical matter, however, linear motion can only be sustained for limited periods of time. For a continuously Doppler shifted signal, some kind of repetitive process of linear motion must be employed.
Imperfections in presently known mechanical means for frequency shifting make it impossible to generate a single Doppler shifted frequency. Instead, currently known systems generate a spectrum, of frequencies having a frequency spacing which is the reciprocal of the period of the time repetitive process. This spectrum has a maximum amplitude at or near the frequency determined by twice the ratio of the velocity component of the reflector (or reflectors) in the direction of wave propogation and the wavelength. The spectral line of maximum amplitude is surrounded by other spectral lines whose amplitudes depend on the "smoothness" of the repetitive process.
A spectrum of frequencies, rather than a single frequency, is undesirable in certain applications, such as in radar applications, where they may lead to ambiguities. One way to eliminate this problem is to space the non-desirable lines far apart from the desired one. This requires a mechanical process of the highest possible repetition rate. The prior art shows several mechanical systems.
One rotating mechanical frequency shifter is describe an article entitled "High Sensitivity HCN Laser Interferometer For Plasma Electron Density Measurements" by D. Veron, which article appears in Volume 10, Number 1 of Optics Communications, dated January 1974. The Veron shifter is, in effect, a large rotating paddle wheel. Each paddle reflects the signal over a small angle of rotation, where its motion can be considered as being linear. Then it is replaced by the next paddle. Although high Doppler offsets may be achieved with high tangential speed of the paddle, many paddles are required to satisfy the linear motion approximation. Consequently, the wheel will have a large diameter and a slow rate of rotation thus producing lines closely spaced in the frequency spectrum.
Another mechanical Doppler frequency shifting scheme is disclosed in an article entitled "CW IR Laser Induced Chemistry, Isotope Separation and Related Laser Technology at NRL" by T. J. Manuccia, which article appears in Laser in Chemistry, pages 210-215, Elsevier Scientific Publication Company, dated 1977. Manuccia's shifter is based upon multiple reflections between a stationary involute spiral cylinder and a multiplicity of mirrors attached to a concentrically rotating cylinder. The transmissive scheme as described by Manuccia, where radiation enters in an axial direction at one end of the cylinder and leaves at the other end of the cylinder, neglects the axial spreading between reflection points as reflections occur at increasing radial distances on the involute cylinder. In addition, it does not provide means to make the Doppler shifted signal phase coherent between cylinder revolutions and hence an undesirable spectral spreading will occur.
U.S. Pat. No. 4,264,141 issued to Karl Guers et al on April 28, 1981, discloses an arrangement of apparatus for frequency shifting a monochromatic narrow bandwidth light beam. Guers et al suggested a device comprised of two or more pairs of stationary and rotating reflectors. Although this device has some desirable features, there is no indication of how to avoid phase discontinuities, and this will result in undesirable spectral broadening.
In Lammers et al U. S. Pat. No. 4,830,479 there is disclosed a rotating Doppler frequency shifter similar to Manuccia's device but with several design improvements. Lammers et al discloses a set of two rotating plane reflectors which direct the source signal to a segmented stationary reflector contoured as the involute of a circle. The segmented reflectors produce a phase continuous signal and are smaller than previous designs due to segmentation. Nevertheless, the Lammers et al device is physically more complicated than is the present invention.
Other background patents of interest are U.S. Pat. Nos. 4,370,141 issued to Krutsch, 4,418,989 issued to McCulla et al, 4,606,031 issued to Beene et al and 4,747,664 issued to Slaughter. In Krutsch, the frequency shifter comprises a self-directive reflective device in the form of a plurality of trihedral corner reflectors. McCulla et al disclose a device for shifting the wavelength of light by reflecting the beam back and forth between a rotating body having a retroreflection corner at opposite ends and a fixed mirror to produce the Doppler shift. Beene at al shows a piezoelectric transducer attached to a laser cavity mirror for fast frequency modulating the output of a laser. Slaughter shows helical vanes which are rotated to provide a reflective marker. None of these patents, or any other of the prior art known to the applicants, suggests the helical frequency changer which is the subject matter of the present invention.